Recent advances in technical issues including poor viewing angle and color definition have been addressed to an acceptable level for liquid crystal displays (LCDs) in mobile devices, monitors and television sets; however, the picture blur of LCDs using a nematic liquid crystal is an issue since the response time of LCD's using a nematic liquid crystal is still insufficiently fast for displaying motion picture quality images. To achieve fast response speed in a LCD, one method is to improve the performance of liquid crystal materials and the other is to improve the device configuration of liquid crystal displays. Several ideas based on modification of device configuration have been developed with the aim of improving the response time including the increase in frame rate. In past decades, several advanced liquid crystal display modes and methods have been developed to solve the viewing angle, for example, in-plane switching (IPS), fringe field switching (FFS), multi-domain vertical alignment (MVA) and patterned vertical alignment (PVA). Recently, through advanced addressing, methods such as scanning backlight, higher-frequency driving and black stripe insertion, LCDs with improved response speed are reported.
In the vertically-aligned (VA) system, the LC molecules are aligned perpendicular to the substrates in the absence of field, thus producing a black image. In this state the polarized light passes through the cell without interruption from the LC molecules and is blocked by the front polarizer. Because there is no twisted structure, the LC molecules are simply switched between vertical and horizontal alignments with a fast response speed. When a voltage is applied across the cell, the nematic LC molecules shift to a horizontal position, producing a white image. The optical transmittance of the vertically aligned nematic LC layer between crossed polarizers can be given as T=sin2 (2φ(V) sin2 (πdΔn(V)/λ where 2φ(V) voltage dependent azimuthal component of the angle between the LC optic axis and the transmission axes of the crossed polarizes and πdΔn(V) is voltage dependent retardation of the LC layer (where δ is the thickness of LC layer, Δn is the birefringence value of LC layer) and 1 is the wavelength of incident light. With an applied voltage, the vertically aligned LC molecules are switched in a direction parallel to substrates making condition of 2φ(V)=π/4 and (πdΔn(V)/λ)=π/2 to maximize the light transmittance, that is T=1. The new multi domain VA modes produce displays with an ultra-high optical contrast between the bright and dark states and wide angle view because the blockage of light transmission is complete at the filed-off state and the viewer see this black from all viewing angles. The switching times of turn-on (rise, τon) and turn-off (decay, τoff) can be described by the following equations: τon=(γ1d2/πKeff) (1/V/Vth)2−1)] and where τoff=(γ1d2/πKeff), where γ1 is the rotational viscosity of LC molecule, Keff is the effective elastic constant (for VA, the Keff is equal to K33) and V and Vth are the applied voltage and threshold voltage, respectively. Due to the weak surface anchoring slow decay time it is insufficient for the above mention VA LCDs to display motion picture quality images.
Simultaneously, various efforts have been carried out to improve the response time of a nematic type LCD based on the development of advanced liquid crystal (LC) material. Further improvements have been reported for LC materials using mixtures with lower viscosity in order to reduce the switching times smaller than one frame time (16 msec); yet, this is still not sufficient to realize motion pictures in a LCD TV, due to the striking difference between cathode-ray-tube (CRT) TV (the so-called impulse driving) and LCD TV (the so-called hold type driving). The hold-type driving scheme of the LCD TV still causes blurring of images even in the theoretical case where the pixels switching voltage is zero.
Incorporating a small amount of polymer network in a liquid crystal to provide the ability of stabilization or modification of the reorientation of LC director in response to applied electric field has been reported by S. H. Kim, L. C. Chien, and L. Komitov, Electro-optical Devices from Polymer-stabilized Molecular Shape Polarity of a Cholesteric Liquid Crystal, SID Digest, 35, 622-625 (2004), S. H. Kim, L. Komitov, L. C. Chien, Short pitch cholesteric electro-optical device stabilized by nonuniform polymer network, Appl. Phys. Lett., 86, 161118 (2005), H. Takatsu, S. Kawakami, G. Sudo, T. Kusumoto, Y. Nagashima, M. Negishi, and T. Matsumoto, Development of Advanced Liquid Crystal Materials, Proceedings of Int. Disp. Res. Conf. IDRC 08 Digest, pp. 29-32 (2008) and M. Bremer, M. Klasen-Memmer, D. Pauluth, K. Tarumi, Novel liquid-crystal materials with negative dielectric anisotropy for TV application, Journal of the SID 14, pp. 514-521 (2006). Additionally, the literature describes a fast switching electro-optical device using an advanced polymer stabilization method where the LC directors are oriented with a surface localized polymer, (M. Wittek, S.-E. Lee, H.-K. Lee, M. Bremer, H. Hirschmann, V. Reiffenrath, B. Rieger, Advanced LC Materials for Ultra-Fast Switching for Active-Matrix-Device (AMD) Applications, Proceedings of Int. Disp. Res. Conf. IDRC 08 Digest, pp. 253-255 (2008)). This device comprised a small amount of a reactive monomer, photoinitiator and liquid crystal mixture; the mixture being filled in a liquid crystal cell with appropriate alignment. Under applied voltage to obtain desired LC alignment, the cell was exposed to ultraviolet (UV) light to polymerize the reactive monomer to obtain a surface localized polymer at both substrates. A similar approach, i.e., a polymer sustained alignment (PSA) was reported in 2004 to improve the light leakage of a MVA mode to obtain high contrast ratio LCDs; however, the need to use an applied voltage to achieve desired polymer protrusion, makes the method cumbersome for manufacturing. Combining both new pixel electrode pattern design and new cell process based on PSA technology, the liquid crystal molecules could well be aligned without protrusion and without ITO slit, resulting in a display with super high static contrast ratio, fast response time and low color washout.
To obtain surface localized polymers with the desired pretilt angle without the need to apply an electric filed during the formation of the polymer, the present invention involves a novel method of forming nano polymer spikes or rods at the substrate surfaces for VA LCDs. Thus, the LCD device of the present invention comprises and liquid crystal cell including a small amount of highly localized surface located polymer, advantageously on both substrates. This polymer is formed on the substrate surfaces as nanospikes or nanorods. In order to obtain these surface localized polymer nanospikes or nanorods with the desired pretilt angle, without the trade off in light transmittance, this invention presents a new method in preparation of surface-polymer-assisted VA LCDs.
The present invention in particular is of advantage in use in In-Plane Switching (IPS), one of the most leading LCD technologies in the world. IPS technology innovatively aligns liquid crystal horizontally to increase the viewing angle and changes the LCD transmittance. This technology was unanimously accepted in its early phase by professionals from all across the globe and was one of the first refinements to produce significant gains in the light-transmissive characteristics in iPad panels. It is a technology that addresses the two main issues of a standard twisted nematic (TN) TFT display color and viewing angle.
In the IPS system, the liquid crystal (LC) molecules are aligned horizontally with an angle of around 12 degree with respect to the electrodes. In this way, the LC molecules are kept parallel to the electrode pair, and thus the glass substrate of the screen. In the absent of field, the LC molecules are aligned parallel to one of the crossed polarizers thus producing a black image. In this state the polarized light passes through the cell without interruption from the LC molecules and is blocked by the front polarizer.
To produce an image, a voltage is applied across the electrodes where the nematic LC molecules are reoriented at an angle to align between the electrodes, normally at 45 degree between crossed polarizers. The electrical field is applied between each end of the crystal molecules—termed a “lateral electric field.” The liquid crystal molecules are weakly anchored to the lower glass substrate, so move more freely into the desired alignment. Because there is no twisted structure the LC molecules are simply switched between dark and bright states in the plane with a fast response speed. IPS improves viewing angles of TFT monitors considerably compared to the TN LCDs due to its character of symmetrical optical retardation at all angle.
Since its introduction in 1996 IPS has gone through a number of advances, with the evolution of Super IPS, Advanced Super IPS, and IPS-Pro. Super IPS was introduced in 1998 to combat the color shift that was still apparent in wide angles of the original IPS screens. With AS-IPS the breakthrough was to move from opaque to transparent electrodes, considerably reducing the amount of power required for an IPS backlight. Also notice the transition to more smooth pixels, giving a cleaner, crisper more continuous image at all angles. IPS-Pro is highly advanced and very expensive, only used in industrial settings where image clarity on a screen is considered critical. Commonly IPS-Pro is used in medical settings, particularly in surgery, but there are other uses in advanced engineering and science that benefit from the clarity and precision of IPS-Pro.